Network Working Group R. Ogier Updates: 5614 October 13, 2011 Internet-Draft Intended status: Experimental Expires: April 15, 2012 Use of OSPF-MDR in Single-Hop Broadcast Networks draft-ietf-ospf-manet-single-hop-mdr-00.txt Abstract RFC 5614 (OSPF-MDR) extends OSPF to support mobile ad hoc networks (MANETs) by specifying its operation on the new OSPF interface of type MANET. This document describes the use of OSPF-MDR in a single-hop broadcast network, which is a special case of a MANET in which each router is a (one-hop) neighbor of each other router. Unlike an OSPF broadcast interface, such an interface can have a different cost associated with each neighbor. The document includes configuration recommendations and simplified mechanisms that can be used in single-hop broadcast networks. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Ogier Expires April 15, 2012 [Page 1] Internet-Draft OSPF-MDR in Single-Hop Broadcast Networks May 2011 1. Introduction OSPF-MDR [RFC5614] specifies an extension of OSPF [RFC2328, RFC5340] to support mobile ad-hoc networks (MANETs) by specifying its operation on the new OSPF interface of type MANET. OSPF-MDR generalizes the Designated Router (DR) to a connected dominating set (CDS) consisting of a typically small subset of routers called MANET Designated Routers (MDRs). Similarly, the Backup Designated Router (BDR) is generalized to a subset of routers called Backup MDRs (BMDRs). MDRs achieve scalability in MANETs similar to the way DRs achieve scalability in broadcast networks: o MDRs have primary responsibility for flooding LSAs. Backup MDRs provide backup flooding when MDRs temporarily fail. o MDRs allow the number of adjacencies to be dramatically reduced, by requiring adjacencies to be formed only between MDR/BMDR routers and their neighbors. In addition, OSPF-MDR has the following features: o MDRs and BMDRs are elected based on information obtained from modified Hello packets received from neighbors. o If adjacency reduction is used (the default), adjacencies are formed between MDRs so as to form a connected subgraph. An option (AdjConnectivity = 2) allows for additional adjacencies to be formed between MDRs/BMDRs to form a biconnected subgraph. o Each non-MDR router becomes adjacent with an MDR called its Parent, and optionally (if AdjConnectivity = 2) becomes adjacent with another MDR or BMDR called its Backup Parent. o Each router advertises connections to its neighbor routers as point-to-point links in its router-LSA. Network-LSAs are not used. o In addition to full-topology LSAs, partial-topology LSAs may be used to reduce the size of router-LSAs. Such LSAs are formatted as standard LSAs, but advertise links to only a subset of neighbors. o Optionally, differential Hellos can be used, which reduce overhead by reporting only changes in neighbor states. This document describes the use of OSPF-MDR in a single-hop broadcast network, which is a special case of a MANET in which each router is a (one-hop) neighbor of each other router. Unlike an OSPF broadcast interface, such an interface can have a different cost associated with each neighbor. An example use case is when the underlying radio system performs layer-2 routing, but has a different number of (layer-2) hops to (layer-3) neighbors. Ogier Expires April 15, 2012 [Page 2] Internet-Draft OSPF-MDR in Single-Hop Broadcast Networks May 2011 Section 2 describes the operation of OSPF-MDR in a single-hop broadcast network with recommended parameter settings. Section 3 describes an alternative procedure which may be used to decide which neighbors on a single-hop broadcast network to advertise in the router-LSA. Section 4 describes a simplified version of the MDR selection algorithm for single-hop networks. The alternative procedure of Section 3 and the simplified algorithm of Section 4 are optional and MUST NOT be used if it is possible for two routers in the network to be more than one hop from each other. 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Operation in a Single-Hop Broadcast Network When OSPF-MDR is used in a single-hop broadcast network, the following parameter settings and options (defined in [RFC5614]) should be used: o AdjConnectivity SHOULD be equal to 2 (biconnected), MAY be equal to 1 (uniconnected), and SHOULD NOT be equal to 0 (full topology). o An adjacency SHOULD be eliminated if neither the router nor the neighbor is an MDR or BMDR (see Section 7.3 of [RFC5614]). o LSAFullness MUST be equal to 4 or 5 if full-topology LSAs are required. (The value 5 is defined in Section 3 of this document.) o LSAFullness MAY be equal to 1 (min-cost LSAs) if full-topology LSAs are not required. This option reduces the number of advertised links while still providing shortest paths. If AdjConnectivity equals 1 or 2 and full-topology LSAs are used, OSPF-MDR running on a single-hop broadcast network has the following properties: o A single MDR is selected, which becomes adjacent with every other router, as in an OSPF broadcast network. o Two BMDRs are selected. This occurs because the MDR selection algorithm ensures that the MDR/BMDR backbone is biconnected. If AdjConnectivity = 2, every non-MDR/BMDR router becomes adjacent with one of the BMDRs in addition to the MDR. o When all adjacencies are fully adjacent, the router-LSA for each router includes point-to-point (type 1) links to all bidirectional Ogier Expires April 15, 2012 [Page 3] Internet-Draft OSPF-MDR in Single-Hop Broadcast Networks May 2011 neighbors (in state 2-Way or greater). 3. Originating Router-LSAs A router running OSPF-MDR with LSAFullness = 4 includes in its router-LSA point-to-point (type 1) links for all fully adjacent neighbors, and for all bidirectional neighbors that are routable. A neighbor is routable if the SPF calculation has produced a route to the neighbor and a flexible quality condition is satisfied. This section describes an alternative procedure which MAY be used instead of the procedure described in Section 6 of [RFC5614], to decide which neighbors on a single-hop broadcast network to advertise in the router-LSA. The alternative procedure will correspond to LSAFullness = 5, and is interoperable with the other choices for LSAFullness. This procedure avoids the need to check whether a neighbor is routable, and thus avoids having to update the set of routable neighbors. If LSAFullness = 5, then the Selected Advertised Neighbor Set (SANS) is the same as specified for LSAFullness = 4, and the following steps are performed instead of the first paragraph of Section 9.4 in [RFC5614]. (1) The MDR includes in its router-LSA a point-to-point (type 1) link for each fully adjacent neighbor. (Note that the MDR becomes adjacent with all of its neighbors.) (2) Each non-MDR router includes in its router-LSA a point-to-point link for each fully adjacent neighbor, and, if the router is fully adjacent with the MDR, for each bidirectional neighbor j such that the MDR's router-LSA includes a link to j. To provide rationale for the above procedure, let i and j be two non-MDR routers. Since the SPF calculation (Section 16.1 of [RFC2328]) allows router i to use router j as a next hop only if router j advertises a link back to router i, routers i and j must both advertise a link to each other in their router-LSAs before either can use the other as a next hop. Therefore, the above procedure for non-MDR routers (Step 2) implies there must exist a path of fully adjacent links between i and j (via the MDR) in both directions before this can happen. The above procedure for non-MDR routers is similar to one described in Section 3.6 of [HYBRID] for non-DR routers. 4. MDR Selection Algorithm The MDR selection algorithm of [RFC5614] simplifies as follows in single-hop networks. The resulting algorithm is similar to the DR Ogier Expires April 15, 2012 [Page 4] Internet-Draft OSPF-MDR in Single-Hop Broadcast Networks May 2011 election algorithm of OSPF, but is slightly different (e.g., two Backup MDRs are selected). The following simplified algorithm is interoperable with the full MDR selection algorithm. Note that lexicographic order is used when comparing tuples of the form (RtrPri, MDR Level, RID). Also note that each router will form adjacencies with its parents and dependent neighbors. In the following, the term "neighbor" refers to a bidirectional neighbor (in state 2-Way or greater). Phase 1 (creating the neighbor connectivity matrix) is not required. Phase 2: MDR Selection (2.1) The set of Dependent Neighbors is initialized to be empty. (2.2) If the router has a larger value of (RtrPri, MDR Level, RID) than all of its (bidirectional) neighbors: the router selects itself as an MDR, selects its BMDR neighbors as Dependent Neighbors if AdjConnectivity = 2, then proceeds to Phase 4. (2.3) Otherwise, if the router's MDR Level is currently MDR, then it is changed to BMDR before executing Phase 3. Phase 3: Backup MDR Selection (3.1) Let Rmax be the neighbor with the largest value of (RtrPri, MDR Level, RID). (3.2) Determine whether or not there exist two neighbors, other than Rmax, with a larger value of (RtrPri, MDR Level, RID) than the router itself. (3.3) If there exist two such neighbors, then the router sets its MDR Level to MDR Other. (3.4) Else, the router sets its MDR Level to BMDR, and if AdjConnectivity = 2, adds Rmax and its MDR/BMDR neighbors as Dependent Neighbors. (3.5) If steps 3.1 through 3.4 resulted in the MDR Level changing from MDR Other to BMDR, then execute Step 2.2 again before proceeding to Phase 4. (This is necessary because running Step 2.2 again can cause the MDR Level to change to MDR.) Phase 4: Parent Selection Each router selects a Parent and (if AdjConnectivity = 2) a Backup Parent for the single-hop broadcast network. The Parent for a non- MDR router will be the MDR. The Backup Parent for an MDR Other, if it exists, will be a BMDR. Each non-MDR router becomes adjacent with Ogier Expires April 15, 2012 [Page 5] Internet-Draft OSPF-MDR in Single-Hop Broadcast Networks May 2011 its Parent and its Backup Parent, if it exists. The parent selection algorithm is already simple, so a simplified version is not given here. The Parent and Backup Parent are analogous to the Designated Router and Backup Designated Router interface data items in OSPF. As in OSPF, these are advertised in the DR and Backup DR fields of each Hello sent on the interface. 5. Security Considerations This document describes the use of OSPF-MDR in a single-hop broadcast network, and raises no security issues in addition to those already covered in [RFC5614]. 6. IANA Considerations This document has no IANA considerations. 7. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, July 2008. [RFC5614] Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET) Extension of OSPF Using Connected Dominating Set (CDS) Flooding", RFC 5614, August 2009. 8. Informative References [HYBRID] Sheth, N., L. Wang, and J. Zhang, "OSPF Hybrid Broadcast and P2MP Interface Type", draft-ietf-ospf-hybrid-bcast-and- p2mp-00.txt, October 2011, work in progress. Author's Address Richard G. Ogier Email: rich.ogier@earthlink.net Ogier Expires April 15, 2012 [Page 6]